units package (WIP)
units manager getters for names and units, exists for unit objects
units manager experimental loadUnits(fileName) method

This method should load units definitions from text file which come
along with "units" command line application.

See "units" Linux application for details
(http://www.gnu.org/software/units/units.html).
units: rename compatible() -> direct()

As compatible() methods defines if one unit can be directly converted to
another unit in one direction (not the other), so it's more precise to
name it direct(). Thus it's more obvious units are built into direct
graph.
documentation
use last "e" to determine number's exponent part
move all exception classes into Unit class
UnitPrefix constructor with name for consistence and compatibility
UnitsManager minor cleanup
UnitsManager.get() method with auto-creation of a unit
test update to use uman.get(class) method
add LogUnit class
update class path vim config option