maxFilter to magFilter in BitmapFontParameter

Currently BitmapFontParameter contains parameters named minFilter and
maxFilter. Second name is misleading, because everywhere else magFilter name
is used, just like in OpenGL itself. I think, that it is better to keep
those unified across whole Libgdx.
Add default constructor to SkinParameter

Without default constructor one cannot create instance of SkinParameter
without specifying null to the constructor. While this might be acceptable
Java practice, it is considered bad practice in Scala, and is reported by
tools that perform style-checking. One could avoid creating of
SkinParameter at all in those cases, but not when loading callback is
to be used.

Because no other Asset Loader Parameter in Libgdx has such limitation,
this commit adds possibility to create default SkinParameter without
specifying null as parameter.
Merge pull request #1000 from aginiewicz/loader-parameters-unification

Loader parameters unification