Add parsing of independent padding attributes

[ataulm]

In our app, we require the ability to set a different value for the right padding (we want 0). Although the material guidelines specify that it's possible to add an offset to indicate that the tab strip is scrollable, there are no written guidelines written that mandate the right padding should equal the left padding.

This change allows users to specify the left and right paddings independently.

---

<com.astuetz.PagerSlidingTabStrip
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    app:pstsPaddingMiddle="true" />

---

<com.astuetz.PagerSlidingTabStrip
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary" />

---

<com.astuetz.PagerSlidingTabStrip
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    android:paddingLeft="48dp"
    android:paddingRight="48dp" />

---

<com.astuetz.PagerSlidingTabStrip
    android:id="@+id/tabs"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    android:paddingLeft="48dp" />

[ataulm]

as per typical Android views, if android:padding is given, it will override android:padding[Side]

[ataulm]

❗ depending on usage, this is a behaviour changing commit:

No change
For existing apps that specify the same padding for left and right, they needn't do anything to maintain behaviour
For existing apps that use app:pstsPaddingMiddle="true" no change is required to maintain behaviour (this seems to override the padding values anyway)

Change
For existing apps that only specify left or right, they must add the other property for no change to be effected
For existing apps that specify different values for left and right padding, they must increase the smaller padding to match the higher one to maintain behaviour

[ouchadam]

👍 for those awesome gif examples

[jpardogo]

Cheers for the PR, I will try to find some time this weekend to review PR's and bugs.

Regarding the this PR. I developed it like that in the first place, to be honest I don't know why I changed it. It is better to have both options. 👍 to the awesome gifs as well

[ataulm]

cool, let me know if you want changes / I don't mind if you checkout my branch and commit on top of it to get it to a state where you're happy to merge in.

[jpardogo]

I will just update the README to let people know what they need to do with their padding attributes now as you well explained in the commit.