ttools: add TapWriter example

This is an example I prepared for Raul Gutierrez at ESAC to demonstrate
how you can generate efficient TAP output from a DB, with reference to
the Gaia archive.  It's relevant for anybody generating VOTable from RDBMS.
ttools: add 2MASS PSC raw ASCII file input handler to examples

The examples sub-package now contains all the classes required to
read data from the DVD/FTP version of the 2MASS Point Source Catalogue.
Previously the PostgresAsciiStarTable class was in the formats
subpackage, but the additional information to actually use it was
not in the repository.  This can act as a template for other custom
large text file readers.
ttools: add AllWise raw ASCII file input handler to examples

An example input handler is added for the AllWise source catalogue
raw ASCII text files.
ttools: fix histogram bugs

Fix a couple of bugs related to out-of-range values in a histogram:
binless histograms were throwing an exception and negative integer
typed X values with a logarithmic X axis were looping infinitely.