str.format() on dhcp.py

Improved string formatting by removing concatenation and replacing with
str.format() usage
Correct Typo in dhcp.py

Correct a typo in a variable name
str.format() on http.py

Improved string formatting by removing concatenation and replacing with
str.format() usage
str.format() on tftp.py

Improved string formatting by removing concatenation and replacing with
str.format() usage
Missing Paren in tftp.py

Added missing paren in tftp.py causing failure to run
Merge pull request #52 from mmattioli/str-format

Replace String Concatenation with str.format()