[IMP] RTL (Right-to-Left) support for UI and reports #35

### summary
solve issue #35 by adding a new variable **web_lang_dir** to qwebcontext which in turn injected in HTML body's class attribute, this variable holds a value equal to *rtl* or *ltr* depending on context language.

the same way used to support rtl for website module, but with a different qwebcontext variable called **website_lang_dir**

### Steps for testing
#### Backend (web_rtl)
1. go to settings -> language
2. edit one of the installed languages (e.g. arabic) direction to *right to left*.
3. go to preference and change your language into the language you choose in step 3.
4. you should see that your UI is RTL layout.

#### Report (report_rtl)
1. change your current language into one of the RTL direction languages.
2. print one of internal layout reports. (e.g. Trial Balance)

for reports that localized against other partners (e.g. sale reoprt).

1. edit one of the partner language into an RTL language.
2. create a sale order for that partner.
3. print the sale order.

#### Frontend (website_rtl)
1. go to settings -> website, and install an RTL language for website.
2. go to frontend, and select that language will display the interface into RTL.