Better DRY in parsing of header variables

Use the type conversion feature of read_pair instead of converting the pairs specifically for the header block
Deal with DXF files containing leading whitespace

Some DXF exporters, Inkscape in particular, add a lot of whitespace to the files
Ignore group code 999

Apparently Inkscape uses 999 for comments
SplineParser now inherits from EntityParser

For more better DRY
Ignore unrecognized sections instead of raising an exception
Initialized for Travis CI and rubygems releases
Require Ruby 2.0 or later