Add body to arguments of buildRequest().

Some web servers require Content-Length Header.
Merge pull request #1 from haruyama/add_body_to_buildRequest_arguments

Add body to arguments of buildRequest().
Add PostMultipart().
Use browser.PostMultipart() when form.enctype == "multipart/form-data"
Add input[checked], "select option" and textarea support.

I want to set formerly-unchecked input[type=checkbox] value and etc. via Input().
This is why fields check in Input() is deleted.
Add a test.
Add definedFileds to Form.

I think again that errors.NewElementNotFound() is useful.
Use map[string]bool type for defiledFields.
Merge pull request #2 from haruyama/multipart

Add multipart/form-data support.
Updated the change log
Updated the readme
Updated the change log
Merge branch 'master' into checked_and_option
Add Form.InputSlice() and Form.CheckBox().

Both have same behavior.
Separate haruyama/surf from headzoo/surf.