Updates gradle plugin version and gradle version to latest

This allows the project to be imported with the latest Android Studio version (0.5.2).
Also removes duplication between build.gradle files
Fixed setAdapter bug

adapter should be mAdapter, otherwise when adding header view, mItemCount will get the original adapter's getCount(), which doesn't count the number of headers and/or footers. Later on in layoutChildren() method it'll throw IllegalStateException, because the number of mItemCount and mAdapter.getCount() will never be equal.
Merge pull request #78 from xrigau/update_build_tools

Merged pull request #78 - Updates gradle plugin version and gradle version to latest
Android build tools 0.9.2 - explicitly tested version perferred over auto updating
Merge pull request #85 from lzanita09/patch-1

Merged pull request #86 - Fixed setAdapter bug
Further fixes for PR #85 - bugs in setAdapter
Updated Gradle to 1.11 - you'll need to use AndroidStudio 0.5.+ to locally build the library
Sample: Added an example of the grid in a fragment in an attempt to replicate #45
Added guard against exception detected in #45
Merge remote-tracking branch 'upstream/master'