feat #1229 output `ariatemplates-latest.js` file in bootstrap build

The bootstrap build now outputs also files named `ariatemplates-latest`,
`atskin-latest` etc. in addition to the versioned names like
`ariatemplates-x.x.x`, `atskin-x.x.x` which allows to include the
generically named file in the development tools (static HTML files etc).
feat #1229 watch task for rebuilding bootstrap code

This commit adds a watch task (runnable via `npm run watch` or `grunt
watch`) which watches all the files in `src` folder and runs
the bootstrap build whenever any of them changes.

This is meant for developer's convenience (re-running tests in the
browser in the development cycle, via `test/test.htm` page)
until JIRA-403 is implemented.

Since 1.6.1, running tests locally requires a bootstrap build
which was added with the migration to noder-js (commit
7743a70).

Close #1229.
doc #1229 update testing documentation