Horizontal padding, left & right views, bug fixes

New features:

• #14 Added horizontal padding. Thanks @bobbyrehm!
• #18 Added support for left and right views. Thanks @RaoulNL!

Updates:

• Updates to Xcode settings and storyboard for Xcode 9.3
• Bumped min deployment target to iOS 10.0
• Modernised constraints in MFTextField

Bug fixes:

• #17 Fix crash when attributed placeholder is nil
• #20 Fix both floating & inline placeholders being shown when text is cleared