Adds more documentation

For me bangsh balances simplicity and power in a great way. I wanted to learn how to use if and took some notes that I converted here to some more extensive documentation. Some things are pretty obvious, but the idea if to allow people to clearly see what bangsh can do for them. Let me know what you think.

added @bellthoven feedback
Merge pull request #15 from davidpelaez/master

Improve README file
Fix module and task lookup failure on `bang test`

The problem is that `./modules` and `./tasks` won't work since the
`PWD` while running `bang test` is `bangsh/bin/`. In order to it work
it requires `$base_name/modules` and `$base_name/tasks`.
Typo
Merge pull request #16 from nerde/master

Fix typo
Fix a bug when displaying `echo` valid options as aliases
Merge pull request #20 from bellthoven/gd-fix-usage

Fix a bug when displaying `echo` valid options as aliases
Move `src/` and `bin` to the root path

`bang.sh` file was moved to `modules/core.sh` file
Add the `run` task

`bang run <file>` runs the `<file>` with the bang.sh environment loaded.
Support for `make install` command

This command symbolic links the `bang` executable to `/usr/local/bin`,
so it is available for being run at any path
Change the samples shebang for the new `bang run` command
Shift the first argument of the `$@`

The first argument is the file being executed, which is now removed so
`bang-app something` will have `something` as `$1` whereas if not removed
it would be `$2` and `$1` would be `bang-app`.
Update travis configuration
Accept an absolute path by `bang run`
Test the `bang run` task