[jnigen] Use dollar signs for nested class name separator

[HosseinYousefi]

The Bar class defined inside the Foo class in Java like class Foo { class Bar {} } used to be generated as Foo_Bar, but we can have a class named Foo_Bar, now we generate it as Foo$Bar.

We can (although very rare – rarer than using underscores) have a class named Foo$Bar that is not nested, this will be generated as Foo$$Bar as all dollar signs that are pre-existing in Java code will be replaced by 2 dollar signs. So we continue not to have any collisions.

I should mention that it's impossible to have both a nested Bar inside Foo and a class named Foo$Bar in the same package in Java because they will share the same binary name of com.package.Foo$Bar, so choosing $ is better than _ because of this as well.

Closes #1516.

[coveralls]

coverage: 88.737% (+0.005%) from 88.732%
when pulling ab242d2 on nested-naming
into 8ea1a9d on main.