Move fancybox constructors upstream for wider use

[alurban]

This PR moves the FancyPlot class and various link constructor utilities upstream to the gwdetchar.io.html module, and uses this to render image HTML on all output pages.

Quick things to note:

• For Lasso in particular, these changes greatly simplify html construction for plots, as the function calls do all the heavy lifting
• This was motivated in part by Lasso's use case being similar to omega scans' (and to hveto's, which will soon import its HTML constructors from gwdetchar)
• The fancybox pop-up functionality is not implemented here, because it involves custom javascript and CSS stylesheets that must be processed at runtime; this is being handled separately by @duncanmmacleod

Test output of affected tools is available below, all made in an environment with python-2.7:

Tool	Output
gwdetchar-lasso-correlation	Link
gwdetchar-overflow	Link
gwdetchar-software-saturations	Link
gwdetchar-scattering	Link

This is also the first PR to start to address #84.

cc @duncanmmacleod, @macedo22, @uberpye, @jrsmith02

[coveralls]

Coverage decreased (-0.003%) to 95.499% when pulling e70d60c on alurban:move-fancyplot into 976bf1d on gwdetchar:master.

[coveralls]

Coverage decreased (-0.003%) to 95.499% when pulling e70d60c on alurban:move-fancyplot into 976bf1d on gwdetchar:master.

[coveralls]

Coverage decreased (-0.003%) to 95.499% when pulling e70d60c on alurban:move-fancyplot into 976bf1d on gwdetchar:master.

[coveralls]

Coverage decreased (-0.003%) to 95.499% when pulling e70d60c on alurban:move-fancyplot into 976bf1d on gwdetchar:master.

[coveralls]

Coverage decreased (-0.003%) to 95.499% when pulling 45fbe40 on alurban:move-fancyplot into bb6fd5f on gwdetchar:master.

[jrsmith02]

This all sounds good to me, and the output pages all look good to me. Thanks @alurban !

[duncanmmacleod]

LGTM