use internal skolem numbering
simplify mkSkolem naming system: don't use $$

Short summary: By default NODEID is appeneded, just continue doing what you
were, just don't add the _$$ at the end.

Long summary:

Before this commit there were four (yes!) ways to specify the names for new
skolems, which result in names as given below

1) mkSkolem("name", ..., SKOLEM_FLAG_DEFAULT)  -> "name_NODEID"
2) mkSkolem("name", ..., SKOLEM_EXACT_NAME)  -> "name"
3) mkSkolem("name_$$", ..., SKOLEM_FLAG_DEFAULT) -> "name_NODEID"
4) mkSkolem("na_$$_me", ..., SKOLEM_FLAG_DEFAULT) -> "na_NODEID_me"

After this commit, only 1) and 2) stay.

90% usage is of 1) or 3), which results in exact same behavior (and
looking at the source code it doesn't look like everyone realized that
the _$$ is just redundant).

Almost no one used 4), which is the only reason to even have $$. Post this
commit if you really want a number in the middle, manually construct the
name and use the SKOLEM_EXACT_NAME flag.
fix warnings in strings/
Eh, what?
optimization
rm undocument/non-working* "feature"

*test of unsigned for negative
attempt to improve CVC4's "parse error" message
travis, please!
Merge remote-tracking branch 'upstream/master' into sets
minor fix
Merge pull request #25 from kbansal/sets

Sets