Strict Error in TGM_Bulk_Installer_Skin class on update to WP3.6

TGM_Bulk_Installer_Skin::before() should be compatible with WP_Upgrader_Skin::before()
TGM_Bulk_Installer_Skin::after() should be compatible with WP_Upgrader_Skin::after()

added in default variable to methods
Merge pull request #123 from hugosaner/develop

Strict Error in TGM_Bulk_Installer_Skin class on update to WP3.6, fixes #122