Implemented ability to close nest levels; aggregate targets are now just regular targets added after close, enabling downstream use
Fixed build error with some pythons
Python 3 fixes

* xrange -> range
* relative import of core module
Swapped out close for pop; calling with no args pops off last nest level. Added test cases
Merged
Replaced xrange with range for python 3 compatibility
Added csmall to .travis.yml
Updated documentation for new aggregate functionality/interface
Leave nest state unchanged on KeyError
Method linking in sphinx docs; wrap some lines
Edits to SCons integration documentation.
Much rehashing of aggregation docs; generally more detailed and with more motivation explanations. doctests also passing.
Aggregation documentation edits

* Initially setting the nest to `nest = SConsWrap(Nest(), ...)`, no
  separate wrap object.
* Some tweaks to pop discussion
* rst syntax

@metasoarous - feel free to reject any of these.
Switched nest1 to level1 etc; a few other minor changes.
Rehashed the scons example to reflect new aggregation functionality.
One final scons example tweak for new aggregation functionality; example running
Fixed conflicts from merging 15-close-nests into master