Added text generation

If a file exists called <templateName>.txt in the same dir as
<templateName>.html, its variables will be compiled with the same context
as the <templateName>.html rendering and the output returned

If such a file does not exists, the text will be autogenerated from the
html using the html-to-text module.

If the option text: false is set, the generateText function will return
null text.
added html-to-text to package.json
Fixed #8
updated README
New tests for text rendering
Removed generateText try block, fs.exist now async

Test text version only if an *out.txt file exists
removed inadvertent file
Merge branch 'txtsupport' of github.com:ckeenan/swig-email-templates

Conflicts:
	package.json
	test/test.js