Added docstring to lambdified functions

If arguments have names, that is used. Otherwise the stack is inspected
to determine if the name can be inferred *with certainty*. If there is
more than one variable pointing to the object, then the name cannot be
determined with certainty, and 'arg_#', where '#' is the number of the
arg is used.
Vector pretty print test fix. Fixes issue #7679.
pretty_symbology: Don't assume the output of U() is not None

U() returns None when unicodedata can't be imported. I'm not sure how to test
this because it's only reproducible if unicodedata from the standard library
can't be imported.  I suppose we could add a hook to test it. It won't be
straight-forward though because the key thing is to make sure it works
correctly at import time.

Fixes #8163.
Added a C code printing method for the sign function.
C code printer now underscores symbols that match reserved words.

This fixes issue #8190 for the C code printer. If the user has a symbol name
that matches a core reserved word in the C language it gets an underscore as a
prefix.
Added a codegen test to check functions with reserved words.
fixed a bug in crypto.rsa_private_key
Return Limit instance if limit function can't compute the limit

Before: PoleError exception was raised
Don't force push to sympy_doc
Deploy the docs as part of the script phase in Travis

This has several advantages:

- If it fails, the build will fail.
- We do not need to rebuild the docs. They are already built from the test
that builds the docs.
- We do not arbitrarily only deploy when the first test in the matrix is
finished. The deploying is now tied to the docs building matrix.
Fix bash syntax in .travis.yml
Fix name of deploy script in .travis.yml
Implement _sage_ helpers for re/im functions

fixes #4230
Docstring for Expr

fixes #4985
Print Limit arrows with unicode

fixes #5255
Merge pull request #8222 from skirpichev/4230-fix

Implement _sage_ helpers for re/im functions
Added recursive printing for sign and fixed tests.
Moved the reserved words check into CodePrinter so all code printers can use it.
Add test for issue 5719, fixes #5719
Merge pull request #8199 from moorepants/c-sign-function

Added a C code printing method for the sign function.